1. Field of the Invention
The present invention relates to bisazo compounds, and to electrophotographic photoconductors comprising an electroconductive support and a photoconductive layer formed thereon containing a bisazo compounds as a charge generating material.
2. Discussion of the Background
In an electrophotographic process, a photoconductor is first exposed to corona charges in the dark so as to electrically charge the surface of the photoconductor uniformly. The thus uniformly charged photoconductor is then exposed to original light images and the exposed portions of the photoconductor selectively become electroconductive and electric charges dissipate therefrom, so that latent electrostatic images corresponding to the original light images are formed on the surface of the photoconductor. The latent electrostatic images are then developed by the so-called toner which comprises a colorant, such as a dye or a pigment, and a binder agent made, for instance, of a polymeric material; thus, visible developed images can be obtained on the photoconductor. It is necessary that photoconductors for use in electrophotography have at least the following fundamental properties: (1) chargeability to a predetermined potential in the dark; (2) minimum electric charge dissipation in the dark; and (3) quick dissipation of electric charges upon exposure to light.
Conventionally, a variety of inorganic and organic electrophotographic photoconductors are known. As inorganic photoconductors for use in electrophotography, there are known types, in which, for instance, selenium, cadmium sulfide or zinc oxide is employed as the photoconductive material.
While the above-mentioned inorganic electrophotographic photoconductors have many advantages over other conventional electrophotographic photoconductors, they have several shortcomings as well in view of the practical use thereof.
For instance, a selenium photoconductor, which is widely used at present, meets the above-mentioned three conditions (1) through (3) fairly well, but it has the shortcomings that it is difficult to work it into the form of a belt due to its poor flexibility. Accordingly its production cost is high, and it is so vulnerable to mechanical shocks that it must be handled with the utmost care.
Cadmium sulfide photoconductors and zinc oxide photoconductors are prepared by dispersing cadmium sulfide or zinc oxide in a binder resin. They can be produced at lower cost as compared with selenium photoconductors and are also used widely in practice. However, the cadmium sulfide and zinc oxide photoconductors are poor in surface smoothness, hardness, tensile strength and wear resistance. Therefore, they are not suitable as photoconductors for use in plain paper copiers in which the photoconductors are used many times in quick repetition.
Recently, organic electrophotographic photoconductors, which are said not to have such shortcomings as in the inorganic electrophotographic photoconductors, have been proposed, and some of them are in fact used in practice. A representative example of such organic electrophotographic photoconductor is an electrophotographic photoconductor comprising a charge transporting complex consisting essentially of poly-N-vinylcarbazole and 2,4,7-trinitrofluorene-9-one.
Although the above-noted organic electrophotographic photoconductor has many advantages over other conventional electrophotographic photoconductors, it is not suitable for use in high speed copying machines, particularly because of its insufficient electrophotographic photosensitivity.
Recently extensive studies have been undertaken directed to the elimination of the above-mentioned shortcomings of the conventional electrophotographic photoconductors. In the course of these studies, double-layered organic electrophotographic photoconductors are proposed as having higher electrophotographic sensitivity and more stable chargeability than other conventional organic electrophotographic photoconductors. Some of them are used in practice in plain paper copiers. Such double-layered photoconductors comprise an electroconductive support, a charge generating layer, which is a thin layer of an organic pigment formed on the electroconductive support, and a charge transporting layer consisting essentially of a charge transporting material, formed on the charge generating layer.
Specific examples of such conventional double-layered organic photoconductor are as follows:
(1) A double-layered organic photoconductor comprising a charge generating layer which is a vacuum-evaporated perylene derivative on an electroconductive support, and a charge transporting layer comprising an oxadiazole derivative as proposed in U.S. Pat. No. 3,871,882.
(2) A double-layered organic photoconductor comprising a charge generating layer which is formed by coating an organic amine solution of Chloro Diane Blue on an electroconductive support and drying the coated solution, and a charge transporting layer consisting essentially of a hydrazone compound as proposed in Japanese Patent Publication No. 55-42380.
(3) A double-layered organic photoconductor comprising a charge generating layer which is formed by coating an organic dispersion of a distyryl benzene type bisazo compound on an electroconductive support and drying the coated dispersion, and a charge transporting layer consisting essentially of a hydrazone compound as proposed in Japanese Laid-Open Patent Application No. 55-84943.
The above double-layered electrophotographic organic photoconductors of course have many advantages over other organic photoconductors, but they still have the following shortcomings as well.
The above-mentioned double-layered organic photoconductor (1) comprising a charge generating layer of a perylene derivative and a charge transporting layer of an oxadiazole derivative can be satisfactorily used in a conventional copying machine. However, its electrophotographic sensitivity is still insufficient for use in high speed copying machine. Furthermore, the perylene derivative used as the charge generating material, upon which the spectral sensitivity of the photoconductor entirely depends, does not exhibit any spectral sensitivity in the entire visible light region. Therefore, it is unsuitable for use in color copier.
The double-layered organic photoconductor (2) comprising a charge generating layer of Chloro Diane Blue and a charge transporting layer of a hydrazone compound has a relatively high electrophotographic photosensitivity. However, when forming the charge transporting layer, an organic amine, such as ethylenediamine, has to be employed, which must be handled with the utmost care because of its caustic nature and the irritating properties of its vapor.
The double-layered organic photoconductor (3) comprising a charge generating layer of a distyryl benzene type bisazo compound and a charge transporting layer consisting essentially of a hydrazone compound has the advantage that its production is easy since the charge generating layer can be formed by merely coating a dispersion of a bisazo compound on the support. However, its electrophotographic photosensitivity is also insufficient for use in high speed copier. Another shortcoming of the photoconductor is that it cannot be used in a laser printer using semi-conductor laser due to its extremely low photosensitivity to semi-conductor laser, in spite of a recent great demand for a photoconductor having high photosensitivity in the wavelength region of semi-conductor laser.
Furthermore, it is known that the following bisazo compounds are useful as charge generating pigments for a double-layered organic photoconductor.
Benzidine-type bisazo compounds as described, for instance, in Japanese Laid-Open Patent Applications No. 47-37543 and No. 52-55643; stilbene-type bisazo compounds as described, for instance, in Japanese Laid-Open Patent Application No. 52-8832; diphenylhexatriene-type bisazo compounds as described, for instance, in Japanese Laid-Open Patent Application No. 58-222152; and diphenylbutadiene-type bisazo compounds as described, for instance, in Japanese Laid-Open Patent Application No. 58-222153.
The double-layered organic photoconductors using the above conventional bisazo compounds are unsuitable for use in high speed copying machines and laser printers due to their insufficient electrophotographic photosensitivities.